Amebiasis and Infection with Free-Living Amebas
Amebiasis is an infection with the intestinal protozoan Entamoeba histolytica. About 90% of infections are asymptomatic, and the remaining 10% produce a spectrum of clinical syndromes ranging from dysentery to abscesses of the liver or other organs.
LIFE CYCLE AND TRANSMISSION
E. histolytica is acquired by ingestion of viable cysts from fecally contaminated water, food, or hands. Food-borne exposure is most prevalent and is particularly likely when food handlers are shedding cysts or food is being grown with feces-contaminated soil, fertilizer, or water. Besides the drinking of contaminated water, less common means of transmission include oral and anal sexual practices and—in rare instances—direct rectal inoculation through colonic irrigation devices. Motile trophozoites are released from cysts in the small intestine and, in most patients, remain as harmless commensals in the large bowel. After encystation, infectious cysts are shed in the stool and can survive for several weeks in a moist environment. In some patients, the trophozoites invade either the bowel mucosa, causing symptomatic colitis, or the bloodstream, causing distant abscesses of the liver, lungs, or brain. The trophozoites may not encyst in patients with active dysentery, and motile hematophagous trophozoites are frequently present in fresh stools. Trophozoites are rapidly killed by exposure to air or stomach acid, however, and therefore cannot transmit infection.
About 10% of the world’s population is infected with Entamoeba, the majority with noninvasive Entamoeba dispar. Amebiasis results from infection with E. histolytica and is the third most common cause of death from parasitic disease (after schistosomiasis and malaria). Invasive colitis and liver abscesses are sevenfold more common among men than among women; this difference has been attributed to a disparity in complement-mediated killing. The wide spectrum of clinical disease caused by Entamoeba is due in part to the differences between these two infecting species. E. histolytica has unique isoenzymes, surface antigens, DNA markers, and virulence properties that distinguish it from other genetically related and morphologically identical species, such as E. dispar and E. moshkovskii.
Most asymptomatic carriers, including men who have sex with men (MSM) and patients with AIDS, harbor E. dispar and have self-limited infections. In this respect, E. dispar is dissimilar to other enteric pathogens such as Cryptosporidium and Cystoisospora belli, which can cause self-limited illnesses in immunocompetent hosts but devastating diarrhea in patients with AIDS. These observations indicate that E. dispar is incapable of causing invasive disease. Unlike E. dispar, E. histolytica can cause invasive disease, as demonstrated in recent reports from Korea, China, and India that suggest higher prevalences of amebic seroconversion, invasive amebiasis, and amebic liver abscesses among HIV-positive than HIV-negative patients. In another study, 10% of asymptomatic patients who were colonized with E. histolytica went on to develop amebic colitis, while ...